Couple of things to check up front -- are you using NuGet >= 2.1? 2.2 was just released yesterday.

Are you using the Portable.MVVMLightLibs package from NuGet?

Do you have a reference to the Portable.MVVMLightLibs in your store app project? If you're deriving from ViewModelBase, then other projects that use it need a reference to the libraries that contain base types. That's regular .NET and has nothing to do with
PCLs. If you add the package to your store app, does that resolve the issue?